In one form of a drive roller unit for driving an article such as a freight container on a conveyor track, as described in U.S. Pat. No. 4,792,037, a drive roller can be driven in rotation by an electric motor means by way of a planetary transmission. The drive roller is mounted rotatably on a guide body by means of which it is movable relative to a base of the unit between a lower rest position and an upper drive position in which it is in engagement with the underside of the article to be driven, the guide body having circular-cylindrical outside peripheral surface regions on which the drive roller is concentrically mounted. The drive roller is substantially in the form of a hollow circular cylinder and the electric motor means, the guide body and the planetary transmission are arranged in the interior of the hollow-cylindrical drive roller. Disposed non-rotatably at the inside of the hollow-cylindrical drive roller is an internal gear ring which is concentric with respect thereto, and the guide body is mounted rotatably about an axis of rotation which is stationary with respect to the base and which is eccentric with respect to its circular-cylindrical outside peripheral surface regions. The stator of the electric motor means, which includes the field winding, is rigidly and non-rotatably connected to a stator housing which serves as the guide body, whose circular-cylindrical outside peripheral surface is arranged eccentrically with respect to the axis of rotation of a drive gear which can be driven by the electric motor means. With that design configuration, when the motor means is switched on, the rotor which is disposed in the interior of the stator initially remains in a stationary condition because of its inherent inertia so that the motor torque acting between the stator and the rotor rotates the stator arrangement and therewith the guide body formed by its housing, around the axis of rotation, which is stationary with respect to the base, of the drive gear which meshes with the internal teeth of the drive roller. As a result, because the guide body is eccentric with respect to the axis of rotation of the drive gear, the drive roller which is mounted on the guide body is moved out of its lower rest position into the upper operative position in which it comes to bear against the bottom of the article to be conveyed, which is disposed above it.
A problem with a design configuration of that kind is that the stator arrangement which includes the field winding must rotate at least over a limited angle of rotation, in operation of the drive roller unit. Therefore, a slip ring assembly of expensive configuration must be provided for the supply of current to the field winding of the stator arrangement. A slip ring assembly of that kind not only complicates the construction of the drive roller unit but it also has a tendency to suffer from fouling and susceptibility to trouble so that short periods between maintenance operations are required.
A further drive roller unit as disclosed in U.S. Pat. No. 3,698,539 has a base or main frame which is to be secured for example to the floor of an aircraft freight compartment, and a mounting frame structure which is mounted at one end pivotably to the main or base frame. In the vicinity of the pivot axis the mounting frame structure carries a motor while at a spacing relative to the pivot axis it has a drive roller which is mounted in such a way as to be rotatable about an axis which is stationary with respect to the mounting frame structure, as well as a cam control arrangement which is connected to the free end of the mounting frame structure and by means of which the mounting frame structure is pivotable relative to the main frame from the rest position into the operative position. The output shaft of the motor is connected by way of a transmission train to the input of a planetary transmission, of which one input is coupled by way of a transmission arrangement to the drive shaft of the drive roller while the other output of the planetary transmission drives the cam control unit.
In the rest condition the mounting frame structure lies on the main frame so that the drive roller is disposed in a lowered condition at a spacing from the bottom of a freight container positioned above same on a roller conveyor track. In order to set the freight container in movement on the roller track, the motor is firstly switched on to drive the input of the planetary transmission. At that time the drive roller is prevented from rotating by means of a slipping clutch with a predetermined braking torque so that the drive moment of the motor inevitably acts by way of the other output of the planetary transmission on the cam control unit which, as a result, by virtue of its cams which bear against the main frame, pivots the mounting frame structure upwardly into the operative position until the drive roller comes into engagement with the underside of the bottom of the freight container. The weight of the freight container causes the movement of the cam control unit to be blocked thereby as the drive roller is pressed against the bottom of the container. As a result the output of the planetary transmission which drives the cam control unit is also blocked so that the other output of the planetary transmission, which is connected to the drive roller, transmits thereto a braking moment which overcomes the braking torque of the slipping clutch so that the drive roller, while being pressed against the bottom of the container, begins to rotate and the container is moved forwardly in a direction which is dependent on the direction of rotation of the drive motor.
A problem which arises with that arrangement in practice is that the entire transmission arrangement from the motor to the drive roller on the one hand and to the cam control assembly on the other hand is of a very expensive design configuration and comprises a large number of components. In addition a cam control assembly is required so that the overall construction includes a large number of mechanically moved components. As a result not only does the arrangement involve a high level of manufacturing costs but it is also of a large size and is of high weight, which is extremely undesirable in regard to aircraft construction. Furthermore the large number of components which are moved relative to each other gives rise to an undesirable susceptibility to wear and short periods of time between maintenance operations are required in order to guarantee the high level of reliability required in operating aircraft.
Furthermore the slipping clutch used in that drive roller unit is what is known as a `fluid-friction clutch`, the operating characteristics of which are highly dependent on temperature. Due to the high temperature differences which may occur in operation in the freight compartment of an aircraft, it is therefore necessary for the maximum torque of the slipping clutch to be selected at such a high level that the torque does not fall below the necessary minimum value, even at the most disadvantageous temperatures. However that gives rise to high force peaks when initiating the rotary movement of the drive roller when it is being pressed against the bottom of the container, and those force peaks act on the floor structure of the freight compartment of the aircraft. That is extremely undesirable in practice as certain floor structure limit loadings should not be exceeded and in addition shock loadings are extremely undesirable.
In particular however the above-described drive roller unit is of large size and the individual components are disposed more or less without protection in juxtaposesd arrangement so that in operation it is not only necessary to reckon on the assembly suffering from fouling and contamination, with the operational faults which are caused thereby, but it is also susceptible to damage caused by objects penetrating into the mechanism or due to the direct action of a force thereon.
A drive roller unit which is similar in terms of design and mode of operation is to be found in U.S. Pat. No. 4,697,694 in which, besides being of large size, the arrangement may also suffer in operation from problems due to foreign bodies penetrating into the mechanism or due to the effect of a force acting thereon.